Neutralization of poliovirus type 1 and other enteroviruses was found to proceed more efficaciously when virus and antibody were in salt-free diluents. When the concentration of NaCl was 400 parts per million (ppm) or less, neutralization was enhanced as much as 700-fold over corresponding samples in isotonic salt solutions. The enhancing effect was pH-dependent, with a wide optimal range between pH 4.5 and 7.0. The increased neutralizing activity in the absence of salts was achieved with purified IgG and not with purified IgM.